In microfluidic-related chemical and biological applications, mixing on the micro scale is important and has been considered one of the most challenging tasks. Specifically, rapid and efficient mixing of small quantities of reactants is useful in areas such as DNA hybridization, cell activation, and enzyme reaction. However, it is difficult to mix fluids in microfluidic systems due to the low Reynolds numbers involved, typically smaller than 10. Under such conditions, micromixing is mostly dominated by diffusion, which is time-consuming and inefficient.
Micromixers can be categorized into passive and active micromixers. Active micromixers use disturbance generated by external energy, such as pressure, electrohydrodynamics, dielectrophoretics, and acoustics, to improve the mixing efficiency. Passive micromixers, on the other hand, rely on molecular diffusion or chaotic advection in the mixing process. They do not require external energy except for the pressure to drive the flow. Among the passive micromixers, the ones based on the chaotic advection principle have drawn more attention because of their higher mixing efficiency than the diffusion type of devices. Since conventional fabrication methods cannot form precisely aligned microchannels where stacking up of 2D patterns, the realization of a complex three-dimensional micromixer has been difficult.